Issue 78

N. Mohan et alii, Frattura ed Integrità Strutturale, 78 (2026) 129-145; DOI: 10.3221/IGF-ESIS.78.08

Further, the SEM micrographs (Fig. 14) reveal the wear mechanism of the boronized samples. The wear mechanism observed at 950°C is similar to the features observed in dry sliding wear tests. At 950°C, for 13Cr4Ni MSS boronized both deformation and fracture of the layer is observed along with the presence of pits and micro-cracks while fracture, tribo-oxidation and fracture are observed in the case of 16Cr-4Ni MSS. At 1000°C, for both grades of MSS severe fracture with micro cracks and pits were seen. The presence of electrolyte (NaCl solution) in the wet sliding wear tests influenced the wear mechanism only at 950°C. Features of deformation, fracture and delamination are characteristic of both adhesive and abrasive wear mechanism and hence a mixed mode of wear mechanism is significant in the 13Cr4Ni MSS boronized at 950°C. However, 16Cr5Ni MSS demonstrated tribo-oxidation behavior along with specific areas fracture indicating an oxidative and abrasive wear mechanism. To ensure, the oxidation phenomenon EDS mapping (Fig. 15) was done on the entire area being seen in the SEM micrograph. The EDS maps clearly indicate a strong presence of oxygen on the worn surface. Initially, the electrolyte (NaCl) solution as a corrosive media attacks the sliding surface, infiltrating through the non-homogeneities on the surface of the boronized layer. However, this attack is mitigated as Cr present on the boronized layer aids in developing a passive film. The repeated sliding action causes this passive layer to breakdown and exposes a fresh area to the electrolyte. The exposed areas react with oxygen, and hydroxide ions present in the NaCl solution and attempt to repassivate. This cycle of passivation breakdown repassivation continually repeats for the entire duration of the tribocorrosion test which is significant in reducing the wear rate of the samples. Also, when two surfaces are sliding against each other at the point of contact there is localized increase in temperature which also leads to the oxidation of the surface. The higher Cr content coupled with the favorable boronizing temperature for better wear resistance might possibly explain the oxidation phenomenon observed in the case of 16Cr5Ni MSS boronized at 950°C. Furthermore, it is difficult to rule out the possibility of oxidation behavior in other boronized samples. The absence of oxidative wear behavior in other boronized samples may be due to the difference in the rates of passivation and re passivation may not be sufficient/effective to promote oxidative wear.

Figure 14: Wear mechanism of boronized MSS at 950°C: (a-c) 13Cr4Ni and (d-f) 16Cr5Ni and at 1000°C of (g-i) 13Cr4Ni and (j l)16Cr5Ni MSS in tribocorrosion.

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